1. Field of the Invention
This invention relates generally to rolling mills in which billets are continuously hot rolled into long products, and is concerned in particular with a method and apparatus for temporarily interrupting the passage of such products between upstream and downstream paths within the mill.
2. Description of the Prior Art
As herein employed, the term “long products” includes bars, rods and the like, and does not include flat products, examples being slabs and strips.
The present invention may be employed to solve problems existing in both nonferrous and ferrous rolling mill environments. For example, in a nonferrous mill employing “up casting” systems, the cast product is delivered upwardly from the casting wheel. This has the advantage of producing high quality products containing minimum amounts of oxides. However, this advantage is, to some extent, offset by slow delivery speeds on the order of 3-10 feet/minute. Problems relating to product heat loss and fire cracking of work rolls preclude the introduction of such slow moving cast products directly into a rolling mill.
There exists a need, therefore, for a method and apparatus that makes it possible to operate upcasting systems with relatively slow delivery speeds in direct sequence with rolling mills having higher take in speeds.
Different problems are encountered in ferrous rolling mills, where typically, billets are heated to an elevated rolling temperature in a furnace. The heated billets are then subjected to continuous rolling in successive roughing, intermediate and finishing sections of the mill, with each mill section being comprised of multiple roll stands. For larger finished products, the entire mill can usually be operated at or close to the maximum capacity of the furnace. However, when the rolling schedule calls for smaller finished products, e.g., 5.5 mm rounds, the capacity of the finishing section is often reduced to well below that of the furnace and the roughing and intermediate mill sections. Under these circumstances, the roughing and intermediate sections can be slowed to match the capacity of the finishing section, but there are limits beyond which this becomes impractical. This is again because acceptable rolling procedure dictates that the heated billets should be introduced into the first stand of the roughing section at a minimum take in speed below which excessive heat loss and fire cracking of the work rolls can occur.
In other cases, for example when rolling high speed tool steels or nickel based alloys, a higher take in speed is required to avoid excessive cooling of the billet, while lower finishing speeds are required to avoid excessive heat generation, which can cause core melting and surface cracking of the product.
The size of the billet can be reduced in order to accommodate rolling at the maximum delivery speed of the mill and at a safe take in speed. However, this would require a new pass design for the roll stands, different guides, a lowering of the coil weight of the finished product, and a reduced production rate. The necessity to store different size billets would create further problems.
Thus, in ferrous mills there also exists a need for a method and apparatus that will make it possible to roll smaller size products while maintaining the mill take in speeds at or above acceptable minimums, without having to reduce the size of the billets being processed, and preferably while continuing to roll at or close to the mill's maximum tonnage rate.